This invention relates generally to highlight color imaging and, more particularly, to the use of inter-image areas for process control such as developability.
In the practice of conventional xerography, it is the general procedure to form bi-level electrostatic latent images on a xerographic surface by first uniformly charging a photoconductive insulating surface or photoreceptor. The charge is selectively dissipated in accordance with a pattern of activating radiation corresponding to original images. The selective dissipation of the charge leaves a latent charge pattern on the imaging surface corresponding to the areas not struck by radiation.
This charge pattern is made visible by developing it with toner. The toner is generally a colored powder which adheres to the charge pattern by electrostatic attraction.
The developed image is then fixed to the imaging surface or is transferred to a receiving substrate such as plain paper to which it is fixed by suitable fusing techniques.
Multi-color imaging has also been accomplished utilizing basic xerographic techniques. In this instance, the foregoing process is essentially repeated for three or four cycles. Thus, the charged photoconductive surface is successively exposed to filtered light images. After each exposure the resultant electrostatic latent image is then developed with toner particles corresponding in color to the subtractive primary of the filtered light image. For example, when a red filter is employed, the electrostatic latent image is developed with toner particles which are cyan in color. The cyan toner powder image is then transferred to the copy sheet. The foregoing process is repeated for a green filtered light image which is developed with magenta toner particles and a blue filtered light image which is developed with yellow toner particles.
Each differently colored toner powdered image is sequentially transferred to the copy sheet in superimposed registration with the powder image previously transferred thereto. In this way, three or more toner powder images are transferred sequentially to the copy sheet. After the toner powder images have been transferred to the copy sheet, they are permanently fused thereto. The foregoing color imaging process is known as full color imaging.
Another color imaging process is known as highlight color imaging. In highlight color imaging two different color developers are customarily employed, usually black and some other color, for example, red.
Whether the imaging process results in single or multiple color images, developability controls are required in order to insure satisfactory operation of the development system. Such controls usually depend on a small developed image or test patch area located in the interdocument or inter-image areas for measuring developer performance. However, in a highlight color imaging system requiring developer switching the interdocument areas of conventional imaging surfaces are unavailable for creating these test patches. This is because developer switching using a conventional imaging surface results in unacceptable test patch development. Due to the time required for completing developer switching a test patch in the inter-image area cannot be satisfactorily developed by the appropriate developer. In fact, the patch is partially developed by both developer systems of a highlight color imaging system.
Developer switching is the process of activating and deactivating two or more developer housings. Switching can be accomplished by the timed disengagement of the developer housings from the charge retentive surface or through reverse rotation of the developer rolls designed for that purpose. Developer housing switching can also be effected through developer housing bias switching. Developer switching is necessary in order to preclude development of one image on the imaging surface with both developers.
Various prior art patents disclose developer switching. Exemplary of these are:
U.S. Pat. No. 4,811,046 granted to Jerome E. May on Mar. 7, 1989 and assigned to the same assignee as the instant invention which discloses a printing apparatus wherein the developer rolls of a selected developer housing or housings can be rotated in a contact preventing direction to permit the development of a tri-level images.
U.S. Pat. No. 4,809,038 granted to Yamamoto et al on Feb. 28, 1989 which discloses a color electrographic apparatus including a latent image forming device for forming plural electrostatic latent images respectively corresponding to image signals of different colors on the surface of a photoconductor, a plurality of toners of different colors each of which has the same polarity as the photoconductor, and a plurality of developing devices which are disposed in the vicinity of the photoconductor and each of which contain a colored toner for respectively developing the latest image without contact with the photoconductor under a direct electric field. A toner image is formed by a repeating cycle of making electric charges on the surface of the photoconductor, activating the latent image forming device so as to light expose an image corresponding to respective image signals of different colors on the surface of the photoconductor and activating a respective developing device so as to develop the exposed image by a respective toner corresponding to the image signal. Even further, a reversal flying preventing device is provided for preventing reversal flying of toner of the image from the photoconductor to one of the developing devices which is not developing as the image passes before the developing device.
U.S. Pat. No. 4,797,703 granted to Vladimir S. Guslits on Jan. 10, 1989 which discloses two development stations each having fixed stops at a predetermined position relative to a toning roller. The stations develop latent images on one surface of a flexible photoconductor, and two parallel rollers are located adjacent the other surface of the photoconductor. One roller or the other roller can deflect the photoconductor into an operative relationship with one or the other of the stations. The apparatus that moves the rollers engages the stops to precisely locate the photoconductor relative to the stations.
U.S. Pat. No. 4,754,301 granted to Kasamura et al on June 28, 1988 which discloses an image forming apparatus includes a plurality of the developing devices for developing the latent image formed on an image bearing member. The plural developing devices are each movable between its developing position for developing the latent image on the image bearing member and its retracted position away from the image bearing member. The image forming apparatus includes a shutter for selectively opening and closing the developing aperture of the developing device of one of the developing devices. The shutter opens and closes the developing aperture of said one of the developing devices in association with movement of the other developing device between the developing position and the retracted position.
The foregoing problem relating to the usage of inter-image areas could be obviated by using a larger charge retentive surface or photoreceptor to provide larger spacing in the interdocument area and/or an area at the edge of the photoreceptor for accommodating the test patch without being affected by developer switching. However, enlarging the photoreceptor reduces the manufacturing yield resulting in a substantial cost increase in an already expensive machine component.